I just really think its a bad idea because of the Storm Path, Storm eyes, BB combo and MRD class traits. Remember that the game mechanics are tied to the weapons a class is using.

This being said that is why a PLD has acces to Rage of Halone, making RDM or BLU of a GLA30 would give them RoA combo and the class main traits.

If a job such as DRK should be a DD, they would need a new main class to give them a full support in their role, a class that will give priority to STR support traits and things that will enhance the final job, I don't see how MRD could do that at this time let alone GLA.

They would need to make changes in the class mechanics for the job to not end up like SCH & SMN.